1. Technical Field
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine provided with a function of controlling feedback of an air-fuel ratio and a function of controlling purge.
2. Background Art
In internal combustion engine for vehicle, a fuel vapor generated from a fuel tank or the like is adsorbed by an activated charcoal of a canister and introduced into an intake system. This process is what is called a purging operation. An exhaust passage is provided with an air-fuel ratio sensor, and feedback of a fuel injection quantity is controlled so that the air-fuel ratio of air-fuel mixture fed to the internal combustion engine may be a theoretical air-fuel ratio. In such an internal combustion engine, an air-fuel ratio feedback correction coefficient varies with 1.0 as a reference value, when no purging operation of the fuel vapor is carried out. However, when starting the purging operation, it becomes necessary to reduce the fuel injection quantity according to the purged quantity of the vapor fuel. Therefore it is essential to establish the air-fuel ratio feedback correction coefficient to be a value smaller than the reference value of 1.0.
In this manner, the air-fuel ratio feedback correction coefficient in the purging operation is controlled to be various values with respect to the reference value depending upon operating conditions of the internal combustion engine, i.e., depending upon ratio of intake air quantity to a purge quantity (hereinafter referred to as a purge rate). For example, if a purge control valve disposed in an introduction passage for the fuel vapor is constantly opened with a fixed degree, the air-fuel ratio feedback correction coefficient is established to be smaller than 1.0 in order to obtain a theoretical mixture ratio. When an acceleration operation of the internal combustion engine is carried out, a negative pressure of an intake pipe is reduced, and the intake air quantity is increased. As a result, the purge rate is reduced and the air-fuel ratio feedback correction coefficient becomes closer to 1.0. A technology for coping with such a change in the purge rate and keeping the air-fuel ratio at a target value is disclosed, for example, in the Japanese Patent Publication (unexamined) No.52139/1993.
The technology disclosed in the Japanese Patent Publication (unexamined) No.52139/1993 is provided with: a first injection quantity correcting means for correcting a fuel injection quantity using an air-fuel ratio feedback correction coefficient; a purge air concentration calculating means for calculating a purge air concentration per target purge rate on the basis of a deviation of the air-fuel ratio feedback correction coefficient caused at the time of carrying out the purge; and a second injection quantity correcting means for reducing the fuel injection quantity at the time of carrying out the purge on the basis of a product obtained by multiplying the purge air concentration by the purge rate. In this prior art, a maximum purge rate, that is a ratio of the purge quantity to the intake air quantity at the time of fully opening the purge control valve, is preliminarily stored in, and a duty ratio of the purge control valve is established to be a target purge rate/a maximum purge rate. Thus the target duty ratio is gradually increased when starting the purge.
In this prior art, the air-fuel ratio feedback correction coefficient is established to be not more than a predetermined value. When the air-fuel ratio is rich, a purge air concentration coefficient is increased by a certain value. Deviation of the air-fuel ratio feedback correction coefficient is caused to reflect on the purge air concentration coefficient at a fixed rate with intervals of fifteen seconds from the beginning of the purge. In this manner, the air-fuel ratio feedback correction coefficient is caused to come closer to 1.0. Accordingly, duty ratio of the purge control valve is controlled so that the purge rate may be constant regardless of the operating conditions of the internal combustion engine, and the fuel injection quantity is corrected on the basis of the product obtained by multiplying the purge rate by the purge air concentration even when the purge rate is changed, whereby the deviation of the air-fuel ratio in a transient period is prevented.
The Japanese Patent Publication (unexamined) No.121264/1996 discloses another technology. In this prior art, an initial value of the purge air concentration coefficient in the intake air is calculated on the basis of a deviation in the air-fuel ratio feedback correction coefficient caused at the time of starting the purge, the purge air concentration coefficient is gradually reduced from the initial value every time when a purge air is purged by a set quantity, and a fuel injection quantity is reduced on the basis of the purge air concentration coefficient at the time of carrying out the purge. In such an arrangement, if the air-fuel ratio feedback correction coefficient deviates from a predetermined range while reducing the purge air concentration coefficient, the reduction in the purge air concentration coefficient is temporarily stopped, thereby controlling the purge air concentration coefficient so as not to deviate from the actual purge air concentration.
The Japanese Patent Publication (unexamined) No. 261038/1996 discloses a further technology. In this prior art, a purge rate is calculated on the basis of a purge quantity and the operating conditions. Air-fuel ratio control means controls an air-fuel ratio feedback correction coefficient for correcting an air-fuel ratio of an air-fuel mixture fed into an internal combustion engine on the basis of a value detected by an air-fuel ratio sensor. Then, a purge air concentration is calculated on the basis of the purge rate and the air-fuel ratio feedback correction coefficient. A purge air concentration correction coefficient is calculated on the basis of the purge rate and the purge air concentration. A fuel injection quantity is calculated on the basis of the air-fuel ratio feedback correction coefficient and the purge air concentration correction coefficient. In this manner, the air-fuel ratio is controlled to be a target air-fuel ratio.
As described above, various techniques have been heretofore proposed about how to control the air-fuel ratio feedback correction coefficient at the time of carrying out purging operation. This air-fuel ratio feedback correction coefficient is an essential coefficient to be used in correcting an air-fuel ratio at the time of carrying out purging operation under the changing operating conditions to a theoretical air-fuel ratio, which is the target value.
However, if any deviation in air-fuel ratio feedback correction coefficient takes places, the deviation is corrected taking a relatively long time. In particular, when introducing a large amount of purge air under the condition of not carrying out any purging operation, a problem exists in that, during the period in which a deviation in air-fuel ratio feedback correction coefficient has been completely corrected, an air-fuel ratio cannot maintain its target value, and a rich condition continues for a predetermined time.
The present invention was made to resolve the above-discussed problems and has an object of obtaining an air-fuel ratio control apparatus for an internal combustion engine being capable of accurately controlling an air-fuel ratio to be a target value at all times when a purge air is introduced. The invention also provides a controlling method.
An air-fuel ratio control apparatus for an internal combustion engine according to the invention comprises: an operating condition detecting means for detecting operating conditions of the internal combustion engine; purge control quantity setting means for introducing a fuel vapor as a purge air into an intake system on the basis of a value detected by the operating condition detecting means and for controlling a quantity of the introduced purge air; an air-fuel ratio sensor for detecting an air-fuel ratio of an air-fuel mixture fed into the internal combustion engine; a fuel injection quantity calculating means for calculating a fuel injection quantity on the basis of an air-fuel ratio feedback correction coefficient that corresponds to a value detected by the air-fuel ratio sensor in order that the air-fuel ratio may be a target value; a purging state detecting means for detecting conditions of the purge air introduced by the purge control quantity setting means; and a purge air introduction correcting means for calculating a correction coefficient to correct the fuel injection quantity on the basis of a signal of the operating condition detecting means and a signal of the purging state detecting means, so that the air-fuel ratio that is changed by starting the introduction of the purge air may be kept at the target value.
As a result, the air-fuel ratio does not deviate from the target value even at the time immediately after starting the introduction of the purge air, and the air-fuel ratio is controlled with accuracy at all times.
It is preferable that the correction coefficient calculated by the purge air introduction correcting means is set according to a length of an introduction stopping period that ends at the time of starting the introduction of the purge air.
As a result, it is possible to correct the air-fuel ratio according to the concentration even when the introduction-stopping period is long and the concentration of the purge air is increased. Consequently, the air-fuel ratio is controlled with accuracy.
It is also preferable that the correction coefficient calculated by the purge air introduction correcting means is set according to the fed quantity of the purge air.
As a result, it is possible to control the air-fuel ratio according to the quantity of the introduced purge air, and the air-fuel ratio is controlled with accuracy.
It is also preferable that the air-fuel ratio control apparatus for an internal combustion engine comprises air-fuel ratio feedback correcting means for calculating the air-fuel ratio feedback correction coefficient, and the correction coefficient given from the purge air introduction correcting means to the fuel injection quantity calculating means is kept for a time equivalent to a delay in correction performed by the air-fuel ratio feedback correcting means.
As a result, it is possible to completely correct the deviation of the air-fuel ratio from the target value caused by the delay in the correction using the air-fuel ratio feedback correction coefficient.
A method for controlling an air-fuel ratio of an internal combustion engine according to the invention comprises the steps of: controlling a fuel injection quantity on the basis of an air-fuel ratio feedback correction coefficient that corresponds to an air-fuel ratio of an air-fuel mixture fed into the internal combustion engine; introducing a fuel vapor as a purge air into an intake system of the internal combustion engine according to operating conditions of the internal combustion engine; and controlling the fuel injection quantity according to a quantity of the introduced purge air in order to control the air-fuel ratio of the air-fuel mixture fed into the internal combustion engine to be a target value; wherein upon starting the introduction of the purge, the fuel injection quantity is reduced by a predetermined quantity during a predetermined period from the starting of the introduction of the purge air in order that the air-fuel ratio of the air-fuel mixture at the time immediately after the introduction of the purge air may be kept at the target value.
As a result, it is possible to obtain a method for controlling an air-fuel ratio of an internal combustion engine in which the air-fuel ratio does not become rich even at the time immediately after starting the operation of the purge control valve and it is possible to correct the deviation of the air-fuel ratio from the target value caused by the delay in the control using the air-fuel ratio feedback correcting means or the like.
It is preferable that a value of the predetermined quantity by which the fuel injection quantity is reduced is set according to a length of an introduction stopping period that ends at the time of starting the introduction of the purge air.
As a result, even if the stopping period of the purging operation is long and the concentration of the purge air is increased, the air-fuel ratio is controlled with accuracy.
It is also preferable that the value of the predetermined quantity by which the fuel injection quantity is reduced is set according to the quantity of the fed purge air.
As a result, it is possible to control the air-fuel ratio according to the quantity of the purge air and control the air-fuel ratio with accuracy.
It is preferable that the predetermined time for which the fuel injection quantity is reduced by the predetermined quantity is established to be equivalent to a delay in control using the air-fuel ratio feedback correction coefficient.
As a result, it is possible to completely correct the deviation of the air-fuel ratio from the target value caused by the delay in the correction using the air-fuel ratio feedback correction coefficient.